The present invention relates generally to universal testing machines and more specifically to methods for applying a multiaxial load to a planar test specimen. Actually, there is no equipment capable to evaluate some behaviour parameters, like tensile, compression or fatigue, in different directions simultaneously. As alternative, the evaluation of these characteristics has been done resorting to the unidirectional dynamometer, doing the assays only in one direction or successively in each direction. In this case the results don't give any indication about the interaction of the multiaxial forces.
In known testing machines of that kind, we observe a frame with an upper transverse member and a base transverse member as well as two interconnecting guide pillars in way to tensile compression and beading testing operations. A central transverse member is displaceable along the guide pillars, while testing devices can be connected between upper and base members and/or between central and base members. The central member is connected to cylinder bodies enclosing the guide pillars (UK Patent Application GB 2276949 A). Because the machine only has one axis, we only can perform uniaxial testing operations, which is the great disadvantage of this machine. Furthermore, only one member can be moved, while the others are static.
Others devices actually can perform biaxial testing operations. The Biaxial Testing Apparatus (U.S. Pat. No. 5,905,205) concerns a rhombus-shaped four-bar linkage that is attached at one vertex to a fixed attachment point and a uniaxial tensile force is applied to the opposite vertex. The test specimen is placed inside the four-bar linkage and is attached to the four-bar linkage by load transfer members connected at one end to the links of the four-bar linkage and at their other end to the gripping jaws holding the testing specimen. The application of the uniaxial tensile force then produces a biaxial tensile force in the test specimen. The particular disadvantage of this approach is the restriction imposed by the physical structure of the device, hindering others testing configurations for beyond the biaxial testing.